In a lake (the word "lake" used herein means a pond, a marsh and the like in addition to a lake), water is hard to move as compared with in the sea or rivers, resulting in stagnating and fails to exhibit a satisfactory self-purifying action, so that pollution of lacustrine water gets worse, to thereby often cause production of algae, odor development and the like.
Purification of lacustrine water thus polluted has been conventionally carried out by arranging an aeration system at a lake, installing a water treating system on the land, or drawing up water by means of a pump from a lake for treatment.